The mechanism of action of carbocyclic nucleoside analogs was investigated in human carcinoma cell line HT-29 and human promyelocytic leukemia cell line HL-60. The 3-deaza analog of neplanocin, c3Nep, was as effective as an inhibitor of S- adenosylhomocysteine (AdoHcy) hydrolase as neplanocin but possessed 1/10 the cytotoxicity, did not effectively inhibit RNA methylation and was not metabolized to an AdoMet-like metabolite. c3Nep was ineffective as a differentiating agent in HL-60 cells. The cyclopentenyl cytidine analog (cCyd) possessed a potent inhibitory effect on CTP synthesis and subsequently, DNA synthesis with moderate cytotoxicity against HT-29 and HL- 60 cells. However, this agent proved to be a very effective differentiating agent for HL-60 cells, probably as a result of its S phase specific effects at noncytotoxic concentrations. In other studies, the adenosine deaminase inhibitor, 2'-deoxycoformycin (dCF), was found to produce complete remissions in hairy cell leukemia in phase II clinical trials.